The present invention is related to packaging for MEMS devices. More particularly, the present invention is related to packaging for relatively large-scale MEMS devices where thermal expansion is problematic.
Microelectromechanical systems (MEMS) were first developed in the 1970's, and commercialized in the 1990's. Generally, MEMS devices are microscopic and are characterized by their ability to interact with the physical world on a small scale. MEMS devices can typically be categorized as either receiving some sort of mechanical input, such as sensors and the like, or devices that generate some sort of mechanical output, such as actuators. Given the extremely small scale of typical MEMS devices, such actuation may be extremely small-scale but still able to be used for a variety of purposes. Examples of MEMS sensors can be used to gather data related to thermal, optical, chemical, and even biological inputs. MEMS-based actuators can also be used for a variety of purposes, including, but not limited to, devices that respond and control the environment by moving, filtering and/or pumping materials.
While MEMS devices have traditionally been very small-scale devices (e.g. the size of a grain of sand) larger MEMS devices have also proven to be extremely useful. For example, in the field of optical communications, the movement provided by MEMS actuators can be extremely useful in providing optical switching, multiplexing and/or selective attenuation. Generally, optical MEMS devices are larger scale than the traditional microscopic structures. However, since these larger MEMS devices are electromechanical in nature, and use existing MEMS technology for fabrication, they are still considered microelectromechanical systems even though they may no longer be “micro.”
Thermal expansion in MEMS devices is well known and appreciated by those skilled in the art. In fact, some MEMS devices employ thermal expansion of dissimilar materials in order to generate actuation. Since MEMS devices are generally microscopic, and since thermal expansion is proportional to the size of the body expanding, expansion of non-actuating portions of MEMS structures has traditionally not been a problem. However, in larger-scale MEMS devices, such as those used in optical communication, the MEMS structure may have a dimension that exceeds 0.5 cm. This is an extremely large-scale MEMS device. Accordingly, the thermal expansion associated with such a large device can adversely impact the mechanical aspect of the MEMS device. For example, in a variable optical attenuator, actuation on the order of micro inches may make a significant difference in optical attenuation. If additional displacement is caused by undesirable displacement due to thermal expansion, non-linearities and/or unpredictable results may occur. Accordingly, there is a desire to minimize thermal effects on large-scale MEMS devices.
Another factor that can cause undesirable displacements in large-scale MEMS devices is the packaging itself. MEMS devices are generally relatively brittle and must be protected from the environment. Accordingly, they are generally disposed within a package of some sort. Due to constraints of size and budget, the packaging material itself is generally formed of a material that is not the same as that of the MEMS structure. Thus, the packaging will generally have a coefficient of thermal expansion that differs from the MEMS material. As the temperature of the entire package/MEMS assembly changes, thermally induced strains occur. Traditional electronics packages (including MEMS packages) generally use a die mount on a single side of the device. Differing coefficients of thermal expansion (CTE) between the die and the package can cause the die to bend. Bending the die for an electronic device is generally not a significant problem since electrical connections will usually accommodate some degree of bending. However, for MEMS devices a small bend or thermally induced strain can cause the MEMS device to malfunction or not perform its intended function as well.
Providing a large-scale MEMS device with improved behavior in response to thermal changes would be extremely useful. Such devices could provide more accurate optical communication devices, such as multiplexers, switches, and attenuators, for example, without significantly increasing the cost of those devices. Further, if the temperature behavior is improved significantly, thermal control of MEMS devices and even temperature sensing of such devices may be obviated.